Two types of tone signals are used for signaling in the PSTN. From a subscriber's premises to the central office, DTMF signaling employs two sinusoidal signals, one from a low frequency set of four “row” tones and one from a higher frequency set of four “column” tones to transmit any of the 10 digital values, the symbols “star” and “pound”, or the additional symbols “A”, “B”, “C”, and “D”. For example, the digit “5” is represented by tones having frequencies of 770 Hz and 1336 Hz. The frequencies used are not spaced at equal increments but rather are deliberately designed to be uncorrelated, i.e., the frequencies are not exact multiples of a common base frequency. A different set of tones spaced at equal intervals of 200 Hz is used by the telephone company for MF signaling among central offices. All possible combinations of two frequencies are used resulting in a 15 distinct tone pairs and unlike DTMF, there are no “row” or “column” associations.
In either situation, the accurate identification of the tones that are present is a process that may have to be carried out under less than ideal conditions. For example, undesired impulse and/or Gaussian noise, induction from power lines as well as components of dial tone, speech and echoes of the tone signals may be present in the telephone loop and should not be mistaken for DTMF signals. In addition, both the accuracy of the generated frequencies as well as their relative amplitudes may vary and the tone signals themselves can be distorted by loss, and non-linear distortion. Various standards have been promulgated such as Telcordia Generic Requirement GR506 Core—Chapter 15 and CCITT recommendations Q.23 and Q.24 in the “Red Book”, Volume VI that detail the bounds of acceptable DTMF signals While GR506 Chapter 16 and ITU Q.455 cover MF tone detection. These standards involve various criteria, such as frequency distortion allowance, twist allowance, noise immunity, guard time, talk-down, talk-off, acceptable signal to noise ratio, and dynamic range, etc. A DTMF detector is required to detect a transmitted signal that has a frequency distortion of less than 1.5% and should not detect any DTMF signals that have frequency distortion of more than 3.5%. The term “twist” refers to the difference, in decibels, between the amplitude of the strongest key pad column tone and the amplitude of the strongest key pad row tone. For example, the AT&T standard requires the twist to be between −8 and +4 decibels. Different noise immunity criteria require that signals must meet a specified signal to noise ratio (SNR) which may range from 12 to 24 decibels. A guard time check criteria requires that tones having a duration greater than 40 milliseconds be detected but those having a duration less than 20 milliseconds not be. Speech immunity refers to the ability of the DTMF detector to accurately distinguish DTMF tone signals from actual speech.
The discrete Fourier transform (DFT) running in a digital signal processor can be useful for detecting the frequency components of waveforms such as tone signals. For example, U.S. Pat. Nos. 5,588,053 and 5,644,634 issued to Xie use the DFT to determine the energy level of the different frequencies present in waveforms being analyzed for the presence of DTMF tone signals. The energy levels of the frequency components are compared with predetermined thresholds to determine whether the tones that are present comply with a preset standard. Because the frequencies employed in DTMF signaling are not exact multiples of a common base frequency, using a fixed number of waveform samples in the DFT to detect any of the DTMF tones that may be present will cause the signal energy of a tone to be distributed across the spectrum possibly leading to an error in tone detection. The Xie patent teaches that the DFT can avoid the energy distribution problem by using different frame lengths of from 168 to 178 samples depending on the frequency being looked for in the sampled waveform. It would be extremely advantageous to be able to use a single frame size having a reasonable number of samples and still obtain detection of the tone pairs within the limits of accuracy required by any of the aforementioned standards.